Many known current printed circuit board constructions require one or more external conductive layers, e.g., circuitry and/or pads for mounting components thereon, and, given today's increased functional demands, a plurality of internal conductive planes, e.g., signal, power and/or ground. To provide effective interconnections between such surface components and the board's conductive circuitry, internal planes and/or pads, the. use of conductive through holes has been adopted wherein several such holes are formed in the board and electrically coupled in a selective manner to internal and external conductive elements. Such holes typically include a conductive, e.g., copper, layer as part thereof which in turn contacts the also typically copper circuitry, pads and/or internal planes.
The term "through hole" or simply "hole" as used herein is meant to include both conductive and non-conductive apertures which may extend entirely through the circuit board or even only partly therethrough (such partial holes are often also called "vias" in the circuit board field), including between only one or more internal layers without being externally exposed. Examples of various circuit board structures which include holes of these types are defined in several published documents, including the following U.S. Letters Patents, issued on the dates identified:
______________________________________ 4,704,791 Chellis et al 11/10/87 5,450,290 Boyko et al 09/12/95 5,487,218 Bhatt et al 01/30/96 5,557,844 Bhatt et al 09/24/96 5,571,593 Arldt et al 11/05/96 5,662,987 Mizumoto et al 09/02/97 ______________________________________
All of these patents are assigned to the same assignee as the present invention and are incorporated herein by reference, as are the teachings of the aforementioned copending applications.
Printed circuit boards of the above type are particularly adapted for having one or more (usually several) electrical components, e.g., semiconductor chips, capacitors, resistors, etc., mounted on an external surface thereof and coupled to various, selected internal conductive planes within the board's dielectric substrate. As demands for increased levels of integration in semiconductor chips and other electrical components continue, parallel demands call for concurrent increased functional capabilities, e.g., increased circuit densities, in printed circuit boards adapted for use with such components. Such demands further emphasize the growing need for more closely spaced electrical components on the board's outer surfaces. For those boards possessing greater functional capabilities and therefore which use several through holes therein, it is highly desirable to position the electrical components directly over the holes to maximize board real estate while assuring a compact, miniaturized final board product.
Increased demands such as those above are particularly significant when it is desirable to couple what are referred to as ball grid array (BGA) or similar components directly onto the board's outer conductive layer(s). These components typically include a plurality of highly dense conductors, e.g., solder ball elements, closely positioned in a fixed pattern on the component's undersurface. This is also the case for directly mounted semiconductor chips (also known in this technology as direct chip attach (or DCA) components) wherein a dense pattern of several minute solder balls are arranged on the chip's small undersurface (that directly facing the underlying circuit board). To successfully accommodate such components, filling of the board's holes with conductive material (called "fill") has been tried, with one or more examples defined in the above-identified issued Letters Patents and copending applications. To accomplish such filling, it is often necessary to utilize a mask or the like with pre-formed, e.g., drilled or punched, apertures therein which coincide with the desired hole pattern. Once provided in the necessary precise alignment, various filler materials have been attempted, including both electrically conductive and non-conductive. One example of such a filler is defined in aforementioned U.S. Pat. No. 5,487,218, wherein the composition is an organic polymeric material with an optional particular filler added thereto to modify the thermal and/or electrical properties of the composition. One excellent reason for such modification is to approximately match the coefficients of thermal expansion of both board substrate material and contained filler.
In an alternative process such as defined in Ser. No. 09/076,649, a mask is not necessary and instead a composite member of a thin support layer and fill material is simply positioned over the substrate's holes and then subjected to sufficient force to rupture the support layer and force the fill into the holes. The support layer is then removed and the filled structure subjected to subsequent processing. Attention is again directed to Ser. No. 08/857,188 for a somewhat similar approach but wherein it is considered necessary to partially cure the fill prior to forcing it into the substrate. Such a partially cured member thus forms what can be referred to as a "plug" which is of sufficient rigidity to enable the subsequent force application. In both of these applications, the filled hole can serve as a platform for receiving subsequent metallization thereon (e.g., to form a circuit line or pad) which forms part of the structure's circuitry. Understandably, the circuit's density is increased as a result of such a feature.
It is believed that a new and unique method of making a circuitized substrate having at least one (and possibly several) holes therein which are filled in an expeditious manner provide the advantages cited above (while also obviating the need for mask members and relatively strong force application) would represent a significant advancement in the art.